1. Field of the Invention
This invention relates to an efficient, highly effective system and method for recovering precious metals contained in a liquid. More specifically, the system employs at least one electrolysis cell unit containing two or more reticulate metal foam cathodes. The system may also contain a means of chemical addition and a filtering means to reduce the particulate content and base metal content contained in the liquid in order to provide a uniform electrolyte flow distribution. The system may be used to recover such precious metals as Au, Ag, and Pt.
2. State of the Art There are many applications where it is necessary or desirable to recover a precious metal from solution. For example, in the manufacture of jewelry, precious metal such as gold or silver are plated onto a base metal. Some of the precious metal accumulates in a rinse solution known as the drag-out rinse during the plating process and would be lost if not recovered from the drag-out rinse. Environmental considerations require the removal of metal pollutants such as mercury, cadmium and silver, from solution to prevent the discharge of metal pollutants into sewers and sewage treatment facilities. Photographic processes require the recovery of silver which accumulates in solution during the photographic development process. It is apparent that the simple, efficient and economic recovery of a variety of metal from solution would be highly desirable and beneficial.
There have been numerous efforts, extending over a long period of time, to provide such a simple, efficient and economic system for recovery of precious metals from solution. These efforts have generally been directed to methods and apparatus for electroplating the metal dissolved in the solution onto a cathode in an electrolytic recovery cell. Such electrolytic recovery cells generally comprise a cathode and anode mounted in spaced apart relationship within a housing and connected to a source of DC current. The housing is positioned in a recovery tank. The solution containing the metal is pumped to the recovery tank and through the recovery cell and the metal plated out on the cathode. Periodically, the cathode is removed from the cell and processed to recover the metal.
One of the major drawbacks in the use of these prior art electrolytic precious metal recovery systems has been the codeposition of unwanted metals together with precious metals on the cathode. A variety of unwanted cation components may be present in the solutions as a result of water hardness, metals dissolved from items being plated, or a gradual build-up of impurities with time. These impurities plate at the cathode, together with precious metal being recovered. A fouling of the cathode surface and loss of product purity can occur.
Another major drawback of these prior art systems has been the construction and method of installation of the cathode used in the recovery cell. It is known that the rate and thoroughness of metal recovery during cathodic deposition is depended upon the cathodic surface area contacting the solution being processed. In order to deal with very dilute solutions or solutions with a high rate of flow, these prior art systems have had to provide electrolytic cell housings which allow for addition of cathodes or enlargement of the size of the cathodes in order to increase cathodic surface area. These provisions for increasing or decreasing cathode surface area are expensive and often involve interrupting the process to accomplish.
Cathodes, which have been employed in cells for recovery of gold from solution, have generally been formed of a metal such as stainless steel, titanium or tantalum wire mesh plated with nickel. A typical example is disclosed in U.S. Pat. No. 4,907,347. To increase the total surface area of the cathode, multiple cathodes have been used, such as disclosed, for example, in U.S. Pat. No. 4,034,422. U.S. Pat. No. 3,331,763 discloses a recovery cell for recovering copper from solution which uses a cathode formed from a plastic sheet laminated between two copper sheets. U.S. Pat. No. 3,141,837 discloses a cathode formed of a substrate of glass or plastic sheet having a metallized surface used for electrodeposition of nickel-iron alloys. U.S. Pat. No. 3,650,925 discloses the use of a cathode formed of an electrically-conductive carbonaceous material such as graphite or carbon used for recovery of various metals from solution.
U.S. Pat. No. 4,276,147 discloses a recovery cell for precious metals that is placed directly into a tank containing the metal solution. The single cathode of the electrolytic cell is of a cylindrical construction formed from a cellular non conductive base layer having an outer layer of conductive material. U.S. Pat. No. 4,384,939 discloses a method and apparatus for the removal of precious metals, such as gold, contained in a liquid in low concentration. The cell unit contains a perforated metal cathode cylinder fitted inside a perforated metal anode cylinder. Both the cathode and anode have screw-type structures which permit electrical connection with the outside of the container. U.S. Pat. No. 4,039,422 discloses a unit for the recovery and removal of metal from solution. The unit employs a series of concentric cylindrical wire mesh electrodes. Furthermore, electrolytic cells having reticulate electrodes have been developed for the recovery of metal ions from various waste streams. For example, U.S. Pat. No. 4,515,672 discloses a reticulate electrode and cell for recovering metal ions from metal plating waste streams and the like. U.S. Pat. No. 4,463,601 discloses a membrane or diaphragm-free, electrolytic process for removal of a significant portion of contaminant metals from waste water. The cell used for this process utilizes reticulate cathodes. U.S. Pat. No. 4,399,020 discloses a membrane or diaphragm free electrolytic cell for the removal of metals present as contaminants in waste water. The metal contaminants are deposited on reticulate cathodes.
None of the foregoing patents disclose a system such as described herein which recovers precious metal from a liquid combined with a unit for chemically treating the waste liquid prior to electrolysis, and the capability of easily changing cathode surface area to deal with changes of solution flow rate and concentrations.